The present invention relates generally to a biomagnetic field apparatus using SQUID (Super-conducting Quantum Interference Device hereafter referred to as SQUID) magnetometers for measuring minute magnetic fields generated from the heart, the brain or the like of an adult, a child, an unborn baby or a fetus or the like. Particularly, the invention relates to an apparatus for extracting a fetal heart beats from waveforms of a measured magnetocardiographic signal of the fetus.
Heretofore, a biomagnetic field measuring apparatus employing SQUID magnetometers has been used in measurement of a minute biomagnetic field (a measured magnetic field is called a cardiac magnetic field (magnetocardiogram) or a brain magnetic field (magnetoencephalogram)) which is formed by an ion current generated concurrently with the myocardial electric activity within a subject (muscular activity in general) or the neuron activity in the brain. The motion of an ion current reflects the electric activity of the subject. By utilizing the motion of an ion current, useful information can be obtained and many researches are being carried out. There have been reports on observations of the automatic nerve activity by obtaining RR-intervals of a fetal heart beats from measured magnetic fields by performing Fast Fourier Transform analysis (FFT analysis), and the clinical effectiveness of analysis of the fetal RR interval has been considered.
Heretofore, normally, when detecting R-wave peaks a certain threshold value was set in advance, and peaks that were higher than this threshold value were detected. (Prior Art 1: Am. J. Obstet. Gynecol., 125, pp. 1115-1120, 1976) Also, there is a report on a case where an autocorrelation method was applied which used an electrocardiographic signal from a fetus. (Prior Art 2: Physiol. Meas., 16, pp. 49-54, 1995)
On the other hand, in the sector of electrocardiography, an apparatus structure has been reported, which collects templates of waveforms considered representing the state of the heart of an examined subject, extracts only necessary data for precise diagnosis of the state associable with the templates from the collected waveforms by using a data-compression algorithm, forms a template waveform for a reduced-data electrocardiographic waveform template by using extracted data, then correlates the reduced-data electrocardiographic waveform template with electro-cardiographic waveforms of the patient, and gives a diagnosis from a correlation result. (Prior Art 3: JP-A-10-211180)
In the prior art 1, since magnetocardiographic waveforms of a fetus are of a very weak signal with only a few of pT at each R-wave peak, if there is a baseline drift with the low frequency due to the mother's breathing, for example, it is impossible to set a threshold value, making it difficult to detect the R waves with a better detection rate. In the prior art 2, there is no description of a method, for example, of detecting R waves with high sensitivity. Further, in the prior art 3, there is no description of a technique for extracting the times of peak occurrence in waveforms from correlation results. Just as described, in the prior art, there has been no means for automatic detection of R waves with a high detection rate, which can perform R-R distance analysis of fetuses. For this reason, no means of detecting R waves has been available to many researchers other than visual detection, and a fairly long time has been taken for R-R analysis of fetuses.
The present invention has as its object to provide a biomagnetic field measuring apparatus capable of accurately detecting the peak times in waveforms (P waves, QRS waves, T waves, etc.) with high precision and stability by the electric activity of a fetal heart from minute magnetic field waveforms produced by the fetal heart.